The inventor required an exceptionally high, and uniform, strength syntactic foam for flotation and as a structural element for a deep sea application. Various commercial foams were tested and all failed to meet a suitable FofS (factor of safety). It is considered that the failure of these materials is in part due to non-uniformity of the materials, leading to variable strength characteristics in different parts of the material. The commercial foams tended to fail on one side first and/or develop serious cracks. As a rather large piece of foam was required in order to provide floatation and structural integrity for the application, the inventor considered the low FofS and the non-uniform strength of commercial foams to be a major disadvantage.
Epoxy composites may be made by combining an epoxy prepolymer, a curing agent and a filler and alloys the resulting mixture to cure. The filler may serve one or more of a number of purposes including increasing stiffness, increasing strength, improving crack resistance and reducing density in the cured composite. If a low level of filler is used, the improvement in properties may be less than required. Also the uncured mixture may have a relatively low viscosity. This can allow partial separation of the filler (due to the different densities of the filler and the epoxy prepolymer) resulting in a cured composite with inhomogeneous properties.
These problems may be addressed by increasing the level of filler in the mixture. This however results in new problems. Increasing the level of filler results in an increase in the viscosity of the uncured mixture. Stirring this mixture in order to achieve a homogeneous product can result in inclusion of large amounts of air, which can generate voids in the cured composite. These voids can adversely affect the physical properties (strength etc.) of the cured composite. Application of a vacuum when mixing can partially address this problem, however the high viscosity of an uncured composite with high filler loading can make it difficult to remove all air bubbles.
There is therefore a need for a process for making epoxy composites which reduces or eliminates voids while allowing for a relatively high filler loading.